WLAN, LAN

Some Theory

Self-help book for WLAN installers/providers

Introduction

To meet the needs of many people interested in building WLAN networks, we have decided to collect in one article a bit of theory and a lot of practical information on quick and effective implementation of wireless networks working in 2.4 GHz and 5 GHz bands (802.11).

WLAN (Wireless Local Area Network) means technology that allows to build wireless data networks with satisfactory parameters and quite large ranges of operation at a comparatively low cost. Additional advantage of this technology is short time needed for its implementation.

The potential of WLANs and its use

wireless access to a local network in home, office, business etc.

wireless access to the Net in public space, e.g. in airports, stations, cafes etc. (hot-spot)

802.11n - standard for transmitting multimedia in homes using MIMO technology, up to 300 Mbps;

802.11e - standard defining QoS - support for high quality of services;

802.16 - WiMax standard for backbone networks of high capacity

Wireless network range

It should be realized that the range of a wireless network depends on many factors; we can have an influence on some of them and the rest is unknown. The range of a wireless network depends on:

1. Factors related to the devices used:

output power (it has been decided by the manufacturer),

cable attenuation (depends on the cable and its length),

gain of the antennas (given by the manufacturer),

sensitivity of the devices (given by the manufacturer).

2. External factors:

attenuation between antennas (can be estimated basing on FSL model);

interferences from other devices (can't be predicted - some additional margin of safety needs to be provided for their compensation),

influence of physical barriers (walls, floors, trees etc.)

So, if we want to know what would be the effective range of our network we have to gather the information mentioned above and carry out simple calculations showed in the further part of this self-help book.

Propagation of radio waves

Fresnel zone

Fresnel zone is one of the most important concepts connected with propagation of electromagnetic waves, which is indispensable to assess parameters of any wireless link. It is the area actively participating in transmission of radio signal energy. Shape of this area is an ellipse in longitudinal section, and circle in cross-section. Radius of this circle is a function of the ratio of distances of the cross-section to the antennas - it has the maximum value in the middle of the link. The importance of the first Fresnel zone comes from the fact that almost all energy of the signal is conveyed via this space.

The shape of Fresnel zone. R1 is the radius of the I zone.

[m];

where:

dkm = d1km+d2km, is the distance between masts in km

d1km - distance from the first antenna in km

d2km - distance from the second antenna in km

Wrongly made installation. The installer didn't ensure mutual visibility of antennas. The radio link does not work.

Another example of wrongly made installation. Presence of barriers in the first Fresnel zone causes that radio link still doesn't work properly.

Installation made correctly. Visibility of antennas and no barriers in the first Fresnel zone. The link has been set up properly.

In practice, no obstacles in the central 60% of the I Fresnel zone guarantee quite minimal power loss.

Relationship of the I Fresnel zone radius as the function of the radio link length,

for systems working in 2.4 GHz and 5 GHz band (table).

Radio link length [km]

60% of the I Fresnel zone radius (0.6R1[m])

2.4 GHz

5 GHz

0.1

1.1

0.7

0.2

1.5

1.0

0.5

2.4

1.6

1

3.4

2.3

2

4.7

3.3

3

5.8

4.0

4

6.7

4.6

5

7.5

5.2

6

8.2

5.7

7

8.9

6.1

8

9.5

6.6

9

10.1

7.0

10

10.6

7.3

Curvature of the Earth

In the case of distances reaching a few kilometers and more, it is needed to include curvature of the ground. For the distance equal 5 km the height of barriers in the middle of link raises by 1m (let's represent the quantity as curvature factor) and for 10km distance - yet 4 m. Antennas should be situated at or slightly above the minimum height fulfilling the condition:

At longer distances, more precise calculations should be performed, based on hypsometric profile of terrain and methods including effects of beam refraction and multiple reflections.

Attenuation of gases and rain

These phenomena are well known and recognized as disadvantageous for proper work of radio systems; in practice they are harmless for 2.4 GHz and 5 GHz WLAN systems.

FSL model and attenuation in free space

The basic problem is to estimate attenuation between transmitter and receiver. When we design outdoor link we can use for this purpose FSL model. It is propagation model of free space, which assumes that:

there is no barrier between transmitter and receiver,

reflected waves don't influence the receiver,

the fist Fresnel zone isn't covered,

there aren't taken into consideration outer interferences and fading.

Attenuation of free space is defined as the loss of signal caused by spherical dispersion of radio waves in space.

FSL for the frequency of 2.4 GHz is determined by the pattern:

Lp [dB] = 100 + 20*log(10) D, where D - distance

FSL for frequency 5.4 GHz is determined by the pattern:

Lp [dB] = 106 + 20*log(10) D, where D - distance

Attenuation of free space and 6dB rule

Radio signal will weaken during its propagation in space, as it moves away from transmission antenna. Determination of attenuation of radio signal is the next step in the designing process.

Distance [km]

Attenuation [dB]

2.4 GHz

5 GHz

0.1

80.4

86.4

0.2

86.4

92.4

0.5

94.4

100.4

1

100.4

106.4

2

106.4

112.4

3

109.9

116.0

4

112.4

118.5

5

114.4

120.4

6

116.0

122.0

7

117.3

123.3

8

118.5

124.5

9

119.5

125.5

10

120.4

126.4

6 dB rule says that double increment in distance causes increase of signal attenuation by 6 dB and double reduction of distance causes increase of signal level by 6dB. Simplicity of this rule allows for easy memorization of the relation. It is enough to remember that in 2.4 GHz band the attenuation at the distance of 1 km is 100 dB.

So, using the 6 dB rule, we will get for distances 2, 4, 8 km attenuation values of 106, 112, 118 dB respectively. For distances 500, 250, 125 m, the attenuation will be 94, 88, 82 dB adequately. The 6 dB rule is also applicable for 5 GHz band and other bands, however, the attenuation in the 5 GHz band for a 1 km distance will be 106 dB, so it means that the 6 dB rule is also applicable in the frequency domain.

Other propagation models

For professional applications engineers use highly sophisticated models, developed for specific conditions and environments, such as:

propagation model with covered Fresnel zone

propagation model including attenuation of walls inside buildings

It is not possible to use such models in amateur calculations.

RSL calculations

The basis for range calculation is creation of the radio link balance in order to obtain the RSL value (received signal level).

Elements of energetic balance of a link:

TSL[dBm] - transmitter signal level (transmitter output power)

RSL[dBm] - received signal level

FSL[dB] - free space loss

GT[dBi] - transmitting antenna gain

GR[dBi] - receiving antenna gain

CLT[dB] - loss of transmitted signal in cable and connectors

CLR[dB] - loss of received signal in cable and connectors

The transmitter sends high frequency signal with TSL[dBm] power level via cable link with CLT[dB] attenuation to transmitting antenna input. Then the antenna radiates the signal and simultaneously focuses it in half power angle, this way achieving amplification effect, given as the gain of the antenna GT[dBi]. Radio wave is attenuated by the value FSL[dB] after it passes distance d [km]. Receiving antenna changes electromagnetic wave into received signal that is increased thanks to the gain of receiving antenna GR[dBi]. After the signal passes through the cable link with CLR[dB] loss to the receiver, it finally has the RSL[dBm] level.

RSL[dBm]= TSL - CLT + GT - FSL + GR - CLR

To secure operation of wireless link against short power drops (or fluctuations), into the calculation there is put additional FM parameter (fade margin). Typical value of this parameter is equal 10 dB.

FM = RSL- RSLFM

RSLFM means the minimum level of the received signal (when a fading occurs). For example, if we want to reach RSLFM = -80 dBm, it is required that the radio link has to obtain RSL = -70 dBm

Our goal is to choose such antennas and equipment that would guarantee the required level of signal (-80 dBm) for most of the time. Most of wireless WLAN devices ensure highest possible speed then.

Selection of devices - an example

Antennas for 2.4 GHz band have usually gain between 7 dBi and 24 dBi. For this band the commonly used cables are H-155 E1170, with attenuation 49.6 dB/100 m, and H-1000 E1192 with attenuation 21.5 dB/100 m.

However, there are already available latest cables up to 6 GHz. These cables are recommended for use in new installations instead of above mentioned cables - Tri-Lan 240 ( E1171) and Tri-Lan 400 WLL E1173.

In 5 GHz band, antennas reach energetic gain from 10 dBi to 32 dBi. So the gain is generally a little higher in comparison to 2.4 GHz band.

As an example - we want to set up radio link over 2 km distance and achieve the best possible parameters of the connection. We use devices with 18 dBm output power. The length of the cable connecting antenna with WLAN device is 7 m for both sides of the link. We can read from the table that for these parameters the sum of GT and GR gain shouldn't be less than 21.65 dB. From the next table we know that we should use ATK8 A7120 antennas.

Caution. Some manufacturers, for marketing purposes, intentionally overestimate energetic gain of antennas. It may cause poor work of radio links using such antennas, drop in transmission speed and even momentary loss of connection. So, the best solution is to use antennas which have been tested in laboratory and which have relevant documents proving these tests. Besides that, existence of a number of neighbor wireless networks may cause degradation of our signal. Therefore it is sometimes better to increase the criterion for FM and assume FM=20 dB.

Transmitter power [dBm]

Type of cable

Length of cable [m]

Range of radio link [km]

0.5

1

2

3

4

6

8

10

15

16

H-155

3

11.38

17.38

23.38

26.88

29.38

32.98

35.48

37.38

40.88

7

15.34

21.34

27.34

30.84

33.34

36.94

39.44

41.34

44.84

15

23.28

29.28

35.28

38.78

41.28

44.88

47.38

49.28

52.78

H-1000

3

9.79

15.79

21.79

25.29

27.79

31.39

33.89

35.79

39.29

7

11.65

17.65

23.65

27.15

29.65

33.25

35.75

37.65

41.15

15

15.36

21.36

27.36

30.86

33.36

36.96

39.46

41.36

44.86

18

H-155

3

9.38

15.38

21.38

24.88

27.38

30.98

33.48

35.38

38.88

7

13.34

19.34

25.34

28.84

31.34

34.94

37.44

39.34

42.84

15

21.28

27.28

33.28

36.78

39.28

42.88

45.38

47.28

50.78

H-1000

3

7.79

13.79

19.79

23.29

25.79

29.39

31.89

33.79

37.29

7

9.65

15.65

21.65

25.15

27.65

31.25

33.75

35.65

39.15

15

13.36

19.36

25.36

28.86

31.36

34.96

37.46

39.36

42.86

20

H-155

3

7.38

13.38

19.38

22.88

25.38

28.98

31.48

33.38

36.88

7

11.34

17.34

23.34

26.84

29.34

32.94

35.44

37.34

40.84

15

19.28

25.28

31.28

34.78

37.28

40.88

43.38

45.28

48.78

H-1000

3

5.79

11.79

17.79

21.29

23.79

27.39

29.89

31.79

35.29

7

7.65

13.65

19.65

23.15

25.65

29.25

31.75

33.65

37.15

15

11.36

17.36

23.36

26.86

29.36

32.96

35.46

37.36

40.86

Table indicating required gain of radio link when there are given: length of the link, transmitter power, type and total length of the used cable

Total required gain of radio link

Recommended type of antenna

14

ATK-P1

22

ATK8

26

ATK16

28

TetraAnt 14 dB

33

Grid 16N

48

Andrew 26T

Above values are theoretical rather, the practical range of links working in 2.4 GHz band wouldn't exceed 2 km. The reason is limitation of radiated power, max. 100 mW EIRP (20 dBm), and usually congested band, which requires to adopt high FM value. As a rule, it is more advantageous to use lower power transmitter and antenna with higher gain than the other way round.

EIRP and choice of devices

Will we break the law using transmitting antenna with very high energetic gain? It should be remarked that regulations don't inform about limits of gain, which can't be exceeded.

So how is it possible that one person can have an antenna with 15 dBi gain, when another breaks the law installing antenna having 10 dBi gain?

The answer for this questions involves the regulations regarding maximal acceptable value of radiated power - EIRP. In many countries the maximum value of EIRP that can be radiated without a special license is 100 mW (20 dBm) in 2.4 GHz band, 200 mW in 5.150-5.250 GHz, and 1 W (30 dBm) in 5.47-5.725 GHz band. But the same levels of EIRP can be achieved in many ways, according to the formulas:

In order not to exceed maximum permissible EIRP value, there have to be selected adequate parameters:

transmitter power,

type and length of cables

gain of antenna.

It's worth stressing again that it is much more advantageous to use lower power transmitter and the antenna with higher gain than the other way round. Why? From the link balance we see that desired radiated power level can be achieved in any way, however the base station isn't only a transmitter, but also a receiver, and then, when it receives signal from a client, no matter the power has been transmitted, only sensitivity of the receiver and gain of the antenna is important. So the gain of antennas is important both during transmitting and during receiving.

The output power level is an important issue, too. Usually it seems that the higher power the better results. But it's not the truth. There is some optimal power level adjusted for the location of clients. Too high transmitting power means needless transmission of our signal beyond the desired area. We can interfere networks working far away from us. We will also be vulnerable to attacks on our network performed by people that are quite a long way from us and thus difficult to spot.

The gains of client stations should also be selected carefully. The client that uses a high gain antenna close to the base station, although receives strong signal, it may during transmission also interfere other, even distant networks. Besides that, it will "see" those networks and what it implies, they will cause additional noise (the higher noise the larger number of errors and lower transmission speed), or it will even share with them the transmission medium - which will also decrease speed. On the other hand, client stations with lower gain, optimal for the specific distance, will only see the base station and won't cause such problems.

Connectors

Most of WLAN devices are equipped with SMA-RP connectors, whilst outdoor antennas have N-type connectors. Using H-155 cable it is needed to terminate it with SMA RP connector on one side, and adequate male or female connector (depending on the antenna) on the other side. If we don't have crimping tool we should choose twist-on connectors. However, crimp-on connectors are preferred for their reliability.

The ways of terminating cables can be found here. After we have prepared the cable, we need to solder the inner wire, then put the central tip on and heat it with a soldering iron.

Selecting a radio channel

2.4 GHz band consists of 13 channels, from which only three are separated from one another. It means that only maximum three WLAN networks can work in a particular area. The installer of a new network should check for free channels before he starts to build the WLAN system. In the case of free channels, he should choose the channel with the lowest level of noise/interference.

Practical tests show that mutual influence of two networks working in the same area depends on the chosen channels and decreases with increasing the space between the channels. When both networks use the same channel, they have up to half of the maximum capacity. The worst case is when they use neighboring channels - their signals make mutually a high level noise that dramatically reduces the effective transfer rate to about 20% of their capacity. 4-channel spacing allows for 70% efficiency. Unfortunately, theoretically independent channels also have certain influence on each other.

Choice of polarization

There are two popular variants of polarization: circle and linear. Circle polarization means that the end of the vector of electricity field draws a circle in space. Circle polarization can be dextrorotary or levorotatory. Radio systems with dextrorotary polarization don't influence systems with levorotatory polarization and vice versa.

Circle polarizations: dextrorotary and levorotatory

In the case of linear polarization, the electric field vector oscillates only in one plane. It is horizontal or vertical plane.

Radio systems with horizontal polarization don't affect systems with vertical polarization, and vice versa, as these polarizations are orthogonal. This feature allows doubling the number of radio systems in one place.

Caution. It is not allowed to use antennas with orthogonal polarization i.e. an antenna with horizontal polarization at one side of a link and with vertical polarization at the other side of the link. When it comes to cooperation of circle polarization antennas with linear polarization antennas - it is possible - however with 3dB power loss.

Noise

In practice, noise is the sum of undesired radio signals, i.e. interferences. Too big level of noise can spoil parameters of any radio link or even make the link unserviceable. Even well balanced radio link may appear to be useless for the reason of high noise level. The designer has no influence on the level of ambient noise. So, how can we protect ourselves against interferences? The simplest way to defend our link against them is finding less congested radio channel. Another way is to select antennas with higher gain, to improve signal to noise ratio (S/N).

The throughput of a wireless link depends on the power level of the received signal and S/N ratio (It is marked as signal strength and signal quality in the drawing.) To reach maximal speed (11 Mbps) the indicator should be in green field (Excellent). If the level of noise increases, even a high value of received signal won't protect us from bandwidth loss.

Effective transfer rate

Because WLAN system is based on CSMA/CA techniques and uses transmission with ACK confirmation, the end user connected to the network via e.g. 11 Mbps link cannot reach real file transfer higher than half the value, i.e. about 5 Mbps. Effective transfer rate of any WLAN link is less than half of the declared bandwidth capacity of the radio channel.

Modes of operation of Access Points

Access Point may work in a few different modes. Each mode is characterized by capability (or not) of supporting specific devices and the features collected in the table:

AP mode

LAN support (number of supported computers)

Support for clients equipped with WLAN cards

Cooperation with APs

Wireless bridge

Yes

No

Wireless Bridge

Multiple bridge

Yes

No

Wireless Bridge

Repeater

No

Yes

Access Point

Access Point

Yes

Yes

Relay Node, AP Client

AP Client

Yes

No

Access Point

Planning of WLAN cells and the service for clients

There are a few ways to cover an area with WLAN signal. It all depends on desired range and capability of the network.

The ways of covering an area with radio signal - sector cells and omnidirectional cell

In the case on the left we have a terrain covered by three APs and three sector antennas. Each AP uses different frequency. In the example on the right we use a single AP with an omnidirectional antenna. The first system can cover 6 times bigger area than the second, and can serve 3 times more subscribers. The cost of connecting a subscriber in each of the systems will depend on the distance from the subscriber to the base station. Subscribers who are located closer to the base station can be equipped with low-gain antennas, which means lower cost.

The size of a cell should be chosen taking into consideration all features of the base station, density of population in the area, and estimated degree of saturation of the market.

In practical solutions, the size of a cell is limited by the shape of the land and various barriers like trees, chimneys, buildings etc.

Devices integrated with antennas

Wireless devices integrating active components (access points) with antennas are still gaining popularity. They are connected with computers directly via twisted pair cables (UTP/FTP), instead of traditional coaxial cables (between wireless modules and passive antennas) with lengths limited to several meters (due to attenuation of RF signal). The UTP/FTP cables can be long up to 30 m (it depends on the power requirements of the device and capacity of the power supply using PoE option). This solution eliminates the difficult problems of running GHz coaxial cables (low flexibility) and their attenuation.

Devices for 2.4 GHz band:

Outdoor access point TL-WA5210G High Power 2.4 GHz. Wireless data transmission can be performed in AP, AP Router, WISP, or WISP Client mode. The device is equipped with a high gain antenna, which, together with the electronic board, is put in a weather-resistant housing. Thanks to the antenna with 12 dBi gain, high output power of the transmitter (27 dBm) and high sensitivity of the receiver, the device allows to create long-range, stable and efficient wireless connections.

ULTIAIR devices have been designed for creating efficient wireless IP CCTV and ISP networks.ULTIAIR series is characterized by short delays and high throughput. The devices operate in unlicensed 5GHz band - no special permissions are needed.